This invention relates in general to systems for sampling electrical and optical signals and relates more particularly to a sampler that utilizes a traveling wave modulator. In a typical prior sampler, an optical waveguide is formed in a substrate parallel to the top planar surface of the substrate and a microstrip delay line is formed on top of the substrate in a direction perpendicular to the direction of the optical waveguide. The substrate is formed of a material, such as LiNbO.sub.3 that exhibits a change in index of refraction when an electric field is applied to the substrate. For a given direction of transmission of light in the substrate, the substrate exhibits an ordinary index of refraction for a first polarization of the light and exhibits an extraordinary index of refraction for the polarization perpendicular to the first. Since the optical signal propagation velocity is equal to the speed of light divided by the index of refraction, each of these two polarization directions represents a distinct light propagation channel with different velocities. In order to avoid breaking up the optical signal into two signals that travel at different velocities in the two different channels, the light in the waveguide is polarized along one of these two directions of polarization.
The delay line is located over the optical waveguide so that electrical signals in the delay line will interact (e.g., electrooptically or electroelastically) with optical signals in the optical waveguide. The electric field produced by the signals in the waveguide affects the propagation velocity of optical signals in the optical waveguide. This shows up as a time delay of an optical signal pulse and as phase modulation of a continuous wave optical signal. This phase modulation is converted to amplitude modulation by interfering this optical signal with a reference optical signal. An optical receiver detects this amplitude modulation. Since the amount of amplitude modulation is proportional to the strength of the electric field in the electrical signal, the optical pulse injected in the optical waveguide samples the electrical signal at the point in the delay line where the optical waveguide passes under the delay line. As the relative delay between the electrical and optical signals is varied, the output signal of the optical receiver maps samples of the waveform under test versus the relative delay.
System sensitivity is inversely proportional to the electro-optic half-wave voltage V.sub..pi. (which is the DC half-wave voltage required to generate a phase shift of .pi. radians) and is directly proportional to the square root of the average power in the optical signal. The output signal from the optical detector can be increased by increasing the energy in the optical pulse. This can be achieved by increasing the power density of the pulse or by increasing the duration of the pulse. Unfortunately, the power density is limited by practical limitations. High power lasers are typically large and expensive. Although small size semiconductor lasers are less expensive and more adaptable to portable systems, a substantial increase in average power can only be achieved by increasing the pulse width which degrades system resolution.
The energy per pulse can also be increased by increasing the pulse width of the optical pulse. However, such increased pulse width also equally decreases the resolution of the sampler. Thus, it would be advantageous to have a new sampler design that improves the sampler sensitivity without these disadvantages.